It had just about 15,000 daily active users (DAUs) at
the time, following a 6 month beta period. That number
ballooned to 500,000 DAUs
in the next 12 months. By April 2015, it had 750,000 DAUs.

Now, Slack claims to have surpassed the 1 million DAU threshold —
in just 16 months — as it announced updated figures for its app
on Wednesday.

Some of the notable figures include:

Over 1.1 million daily active users

Over 300,000 paid seats (people paying $6.67/month or
$12.50/month to use it)

Over $25 million annual recurring revenue (the one year
extrapolated sales figure based on current month's sales)

180 employees; 900,000 integrations in Slack

The fact that it added another 350,000 daily active users in just
about two months, a 47% increase, is quite impressive. During
that time, it's also added 100,000 new paid seats, which
contributed to its slightly increased $25 million ARR.

That's probably not enough to make sense of its
latest $2.8 billion valuation, but its unprecedented growth —
which it seems to have no problem sustaining — is more than
enough for venture capitalists to justify its investments. Slack
has raised a total of $340 million from Silicon Valley bigwigs,
including Kleiner Perkins, Social+Capital Partnership, and
Andreessen Horowitz.

One reason people love using Slack is its vast integration
capability with third party apps, like Twitter, Zendesk, and
Stripe. It can send automatic notifications of customer support
tickets that come through Zendesk, for example, allowing users to
keep all of their communications in one single platform.

As part of its effort to become an even bigger communication
platform, Slack also hired a former Twitter and Google engineer
April Underwood as its head of platform Wednesday. She will
oversee the app's platform partnerships, API integrations, and
developer relations.

Underwood's hiring also underscores Slack's move towards a more
robust product that can cater to much bigger, serious enterprise
customers. Earlier this month, it made
a bunch of updates to improve its speed and fix some bugs. In
November, it also
hired Bill Macaitis, its first chief marketing officer, away
from Zendesk. Slack plans to roll out an enterprise tier package
by this year, which would cost about $48 a month for each user.